In cardiology, imaging techniques are common which need to record over several cardiac cycles in order to obtain sufficient spatial and temporal resolution. For this reason, images and heartbeat are recorded synchronously, while the images are recorded with a fixed repetition rate.
Especially in echocardiography, currently such methods are common, which select those images that were taken within one cardiac interval from a sequence of consecutive 2D images or 3D volumes. In the following, without loss of generality it is assumed that a cardiac cycle extends accurately from one R-spike of the ECG signal to the next.
The patent DE 197 32 125 relates to a method for recording ultrasound images of moving objects, wherein during the largest movements of the object no or only a few pictures, and at times of less movement of the object several pictures are taken. By coupling the recording times with the ECG of the creature, the systole is omitted, which means that during a short period no pictures are taken, and afterwards multiple images are recorded during the diastole according to the electrocardiogram. This is also possible by continuously recording the organ, during which the data processing system selects suitable images and processes the desired images. Such medical imaging methods or ultrasonic imaging procedures based on electrocardiographic information are also known from U.S. Pat. No. 6,673,017 and U.S. patent application U.S. 2005/0238216. ECG-triggered imaging is described in U.S. Pat. No. 5,159,931. A disadvantage of these methods is that they usually lag behind the movement cycle, i.e. the dead time between the processing of the cardiac cycle and the implementation for the control of the recording times is relatively large.
U.S. Pat. No. 6,966,878 describes a method for recording and processing a volume scan of a periodically moving object. For that purpose a volume scan at a periodically moving object is performed, wherein within the volume scan, a time interval of the periodic motion of the object is identified and afterwards the volume scan is regrouped based on this time interval.
Since the movements of the moving object, such as the repetition of the heartbeat, and the recording times of the images are often asynchronous, and since the movements themselves are not regular, the time of the first recording in each cycle, based on the particular state of motion of the moving object, can be variable. Depending on the acquisition/recording method (nearest or subsequent recording) the range of variation is [−Δt/2 . . . Δt/2] or [0 . . . Δt], where Δt is the time interval between consecutive recordings. Especially when imaging rapidly and/or irregularly moving objects, this temporal variation leads to artefacts in the subsequent spatial reconstruction.
Therefore pictures of objects that produce a motion blur due to their own movements are usually taken in synchrony to that movement. Without the synchronization to the corresponding movement of the object (stroboscopic imaging), a blurred image, or an image of the object in each of its states of motion, is the result. When recording images in synchrony to the motion of the object (an exact synchronization is currently not possible with the resources of the state of the art), one image is created for each state of motion of the moving object. One example is the three-dimensional ultrasound imaging of the heart. The corresponding images of the heart show successively all stages (phases) between contraction (systole) and relaxation (diastole) of the heart. The successive appearance of individual photographs of the heart corresponds to a four-dimensional representation of the heart, with the fourth dimension representing the corresponding movement of the heart (timeline).
The solution known from DE 197 32 125 therefore relies on combining the recording times of the moving object with the ECG of the creature. That is done e.g. by scanning the patient with ECG electrodes. From the measured ECG a R-spike—or another distinguished point—is determined. The temporal position of the R-spike is used to synchronize continuously taken ultrasound images of the heart to the cardiac cycle, or the acquisition of ultrasound data is started with the detection of the R-spike. The end of the cardiac cycle is then obtained either by using a moving average of the cardiac cycle duration (gained from the R-spikes over time) or the subsequent R-spike is used as the end of the current cardiac cycle. If no ECG is available, the image data must be cut/trimmed manually to a cardiac cycle by the doctor under visual control. Thus, the cycle length can be determined retrospectively from the image content manually or automatically (with appropriate image recognition system).
The ECG electrodes may also peel away, in particular during the stress levels (stress test), so that the ECG cannot be detected and thus the recording of the data is hindered. Furthermore, using electrodes might be costly, since the electrodes themselves are not cheap and their attachment to the patient takes time and creates costs for staff and reduces the throughput of examinations. In case that problems occur with the ECG recording, the manual trimming of the image data causes considerable amount of work by a highly specialized worker.
The solution known from U.S. Pat. No. 6,966,878 B2 is based on the determination of a time interval from the image data, for instance on the basis of certain intensity values or the intensity curve, of a time-based acquisition of ultrasound images during an examination. The time interval is the grid, in which the continuous image data stream is divided into cardiac cycles, i.e. the image data is assigned to a position in the cardiac cycle. The position of the grid, i.e. the phase, is left aside here.
However, a better spatial resolution is desirable in particular with regard to different stress levels. Thus artefacts occurring when recording images in cardiology are prevented/reduced. This is advantageous especially if the object does not behave strictly periodically but only quasi-periodically or even non-periodically, as is often the case when cardiac defects occur. Furthermore, the disadvantages caused by the use of electrodes for ECG are avoided by the synchronization without ECG.
DE 10 2005 014 445 A1 discloses a medical imaging system comprising a control device, at least one image acquisition device and a trigger device. The trigger device, alike the device described in DE 197 32 125, detects the phase angles of the object, for instance the heart, and sequentially sends multiple trigger impulses to the control unit. The trigger pulses all correspond with a predetermined phase position (reference phase) of the object, for instance the start of the systole of the heart. For the typical case where the object is a beating heart, an ECG-triggering is used.
The trigger device of DE 10 2005 014 445 A1 detects only the phase of the object. The starting times for image recording are either identical to the times when activating trigger signals occur or have a predetermined delay time to these times. The detection of the sequences of images is terminated when reaching a stop time point. This means that in certain circumstances, the sequences do not have the same number of images. Although the numbers are about the same, they are not exactly the same. They normally vary by one or two pictures. The control device must therefore modify those sequences which contain a smaller number of images. Furthermore, those images recorded with the same temporal distance to the starting time do not match to the same phase, because they are recorded or evaluated with a constant temporal distance between each other, and thus changing speed of the moving object is not taken into account and artefacts occur.